Population Synthesis Models for Late Buildup of the Red Sequence

We present population synthesis models designed to represent the starformation histories of L* red sequence galaxies (RSGs). Earlier work has shownthat single-burst stellar populations (SSPs) are unable to match Balmer linestrengths simultaneously at high and low redshift. We therefore consideralternative star formation histories in which RSGs contain intermediate-agedstars even at late epochs. The models are compared to Balmer Hdelta absorptionstrengths, U-B color data, and the number density of red sequence galaxies fromz=1 to z=0. We find that quenched models (which consist of constant starformation histories truncated at regularly-spaced intervals) average to an RSGpopulation that matches the data well, showing slow evolution in color andBalmer line strength and a rise in number density by a factor of a few afterz=1. The data are best fit by a turn-on of quenching at redshifts z=1.5-2.Comment: 4 pages, 3 figures, to appear in the Astrophysical Journal Letter